The FFT analyzer has rapidly gained popularity for making frequency response measurements on mechanical systems. One reason for this popularity is the ability of these analyzers to quickly and easily measure frequency response functions using impulsive excitation generated by an impact hammer. For lightly damped, linear structures, this low energy excitation technique provides good results, which accurately predict the dynamic behavior of the structure under higher level inputs.
However, many real-life structures exhibit a considerable degree of non-linearity and/or high damping. For example, bolted or riveted joints produce frictional damping and “gap” discontinuances; elastomeric isolation mounts are often highly nonlinear. In such cases, the frequency response function can change dramatically depending on the amplitude of the input, and so a useful measurement requires higher energy excitation techniques which approximate the dynamic levels actually observed in the operating environment.
These techniques involve the use of an exciter which is able to reproduce a desired force waveform, usually either random or sinusoidal nature.
A properly designed hydraulic exciter system provides an excellent general purpose tool for high energy structural excitation. Some of the key features which may be important for a successful frequency response are: